1. Field of the Invention
The present invention relates to oxidatively drying coating compositions containing oxocarboxylic acids or derivatives thereof and optionally dioxo compounds as anti-skinning agents and to improve drying of the resulting coatings (lacquer films).
2. Description of the Prior Art
Oils and binders are known that are converted from the liquid phase to the solid phase by oxidative crosslinking under the action of oxygen (preferably from the air) at room temperature or at higher temperatures in the presence of drying agents, for example, metal soaps of transition metals. Solutions of these drying agents are known as "siccatives." One difficulty of these oils and binders is that a skin may form on their surface during storage (in open or sealed containers). This crosslinking, which proceeds before the actual application of the lacquer, is highly undesirable. For example, it complicates handling of the lacquer and very frequently prevents the uniform distribution of the siccatives. One cause of skinning is the accumulation of siccatives at the surface. This is detrimental because the incorporation of the siccatives into the lacquer skin removes the necessary active siccatives from the remainder of the lacquer, which severely delays drying.
Skinning is also unwanted and disadvantageous in an applied lacquer film. Excessively rapid drying at the lacquer surface prevents uniform hard drying of the film because the underlying layers of the film are shielded from the oxygen necessary for drying. Uniform distribution of the oxygen is prevented by the excessively rapid surface drying resulting in levelling problems, shrinkage, adhesion problems and/or overall inadequate film hardness of the lacquer films.
In accordance with the prior art it is necessary to add substances (anti-oxidants or anti-skinning agents) to the coating compositions which inhibit the reaction with atmospheric oxygen at the (lacquer) liquid surface, i.e., in the container and in the applied film. A list of these known compounds is set forth in H. Kittel, Lehrbuch der Lacke und Beschichtung, Colomb Verlag, 1976.
The preferred anti-skinning agents are phenolic compounds and oximes. However, phenolic anti-skinning agents considerably delay surface drying, such that they may only be considered for specific coating compositions. Due to their volatility oximes, such as methyl ethyl ketoxime, only slightly delay surface drying. The disadvantage of oximes, which are today widely used, is their toxicity. Animal studies have indicated a risk of carcinogenicity on exposure by inhalation (MAK [German maximum admissible concentration]) list 1997). Therefore, elaborate personal protection measures are required when applying lacquers containing oximes as anti-skinning agents.
An object of the present invention is to provide anti-skinning agents which 1) either do not or only very slightly delay surface drying, 2) allow for very good hard drying of the lacquer and 3) have no disadvantageous toxicological properties. An additional object of the present invention is to provide anti-skinning agents which may be incorporated into a wide variety of oxidatively drying lacquers. Another object is to provide additives which, by virtue of their physical properties, may straightforwardly be used in suitable lacquer formulations.
These objects may be achieved with the compounds according to the present invention, which are described hereinafter.